Genomic DNA purified from the R strain of Rickettsia rickettsii was cloned into plasmid and bacteriophage cloning vectors. Escherichia coli cells harboring either recombinant plasmids or infected with recombinant phage are screened for the production of rickettsial antigens by their reactivity with polyclonal sera raised against intact R. rickettsii. Thus far, three recombinant plasmids and one recombinant phage, which encode rickettsial antigens, have been identified. The insert from one of the recombinant plasmids was subcloned into pUC8 and pUC9. Cells harboring these plasmids have been used as vaccines in mice in efforts to prevent the death of mice caused by intravenous injection with viable R. rickettsii. Cells harboring the pUC8 subclone offered some protection as 50% of the mice vaccinated in this group survived challenge with a 2 LD50 of R. rickettsii.